1. Field of the Invention
The present invention relates to an optical pick-up and an optical head utilizing the optical pick-up, in particular, an optical pick-up for use in an optical recording/reproducing apparatus for recording and reproducing the information to be recorded on an optical recording medium such as an optical disk by use of the reflection light rays reflected thereon, and an optical head utilizing the above-mentioned optical pick-up for recording, reproducing, and erasing the information by use of the reflected light rays from the optical information recording medium.
2. Description of the Prior Art
An optical disk apparatus has a large capacity, compared with a hard disk apparatus or a magnetic tape apparatus, etc. However, since the optical disk apparatus has not yet accomplished higher access speed, it has fallen behind the magnetic recording apparatus in field of the high-speed read-out devices. Various attempts have been made to improve the access time. As to the approach by utilization of the optical system, its a main stream to aim is realizing a small-sized and light-weight optical pick-up by reducing the number of employed optical parts.
And further, research of the above matters has been vigorous, in the field of "write-once", CD (Compact Disk), and the like. One of the to the adoption of a diffraction grating. Since the diffraction grating can concentrate the functions of the individual optical parts, it is possible to effectively reduce the number of optical parts. The works by Lee et al. are well known in the art.
The diffraction grating devised by Lee et al. concentrates the functions a beam splitter, cylindrical lens, or the like. And structurally, a, it is made of a thin plate and suitable for being small-sized. FIG. 28 is an explanatory view showing an optical pick-up employing the above diffraction grating.
In FIG. 28, the outgoing light rays emitted from a semiconductor laser (LD) 5 enter a diffraction grating 4 as incident light rays thereto.
The rate of the transmission of the light rays passing through the diffraction grating 4 is almost 70%. The diffracted light rays occur in several orders. The maximum diffraction efficiency of the .+-.1-order light rays utilized for detection is almost 10%. The transmission light rays passing through the diffraction grating 4 are collimated by a collimation lens 3. On this occasion, the round portion of the outgoing light rays emitted from the semiconductor laser 5 is drawn out by the collimation lens 3, and thereby beam-forming is performed. (In the other hand, the collimated light rays are focusedly radiated on an optical disk 1 by an objective lens 2, and the reflection light rays reflected on the optical disk 1 pass through the same optical route and arrive at the diffraction grating 4.
The diffracted light rays are employed for detecting the signal emitted from the above-mentioned optical pick-up. The pitch of the diffraction grating 4 is treated by charping for monotonously increasing or decreasing the pitch thereof depending on its position. (Such grating is called "a charped diffraction grating".) The diffracted light rays obtain same effect as in the case of passing through the cylindrical lens. As shown in FIG. 29, regarding the diffracted light rays, there exists a difference in the function of the convex/concave lens by the action of the .+-.1-order light rays. It is permitted to use any one of both diffracted light rays employed for detecting the signal. Concerning the optical pick-up shown in FIG. 28, the case of employing the convex lens is explained.
The incident light rays entering the diffraction grating 4 have an astigmatism, since the function of the above-mentioned convex lens exerts an influence on the focused light rays. Consequently, the focused light rays emitted from the collimation lens 3 enter the branched diffraciton grating 4 as the incident light rays, and thereby the diffracted light rays show the astigmatism. The focus signal Fo can be detected by use of a four-divisional light-receiving element (in general, photo-diode; PD), utilizing the astigmatism method which is well-known conventionally.
Namely, in the case of employing the photo-diode PD, the spot of the light rays varies as shown in FIG. 30 in accordance with the position of the optical disk 1 (near or far away), and the focus signal Fo is the one as shown in FIG. 31.
The track signal Tr can be obtained also by the four-divisional light-receiving element PD. And further, the recording signal Rf can be obtained from the total output. The respective signals; Fo, Tr, and Rf are expressed by the following equations (1)-(3) from FIG. 30: EQU Fo=(A+C)-(B+D) (1) EQU Tr=(A+D)-(B+C) (2) EQU Rf=A+B+C+D (3)
Next, the charping is executed on the grid of the diffraction grating in order to detect the focus signal. FIG. 8 shows an example of causing a part of the diffraction grating to have a distribution of pitch and detecting even the track signal. (Refer to "Application Physics Academic Meeting 1988, Spring Preliminary Drafts Assembly, 29p-ZQ-11.)
In FIG. 5, regarding the distribution of the diffraction grating 40, the grating is constructed with a parallel grating treated with the charp (the tendons thereof are parallel with each other) and a couple of slit-state diffraction gratings, the grating direction of which is inclined to the direction of the above parallel grating. In such construction, the slit-state diffraction grating distributes the track patterns appearing on the surface of the diffraction grating onto the two photo-diodes (PDs) as shown by TE in FIG. 32.
Parallel light rays enter the diffraction grating 40 as incident light rays. And further, a lens 30 is disposed at the downstream side of the diffraction grating 40. In such construction, the track signal can be obtained from the difference of the output signals emitted by two PDs represented by TE, utilizing the push-pull method which is the conventional method of detecting the track signal. Furthermore, the focus signal can be obtained by the four-divisional PDs represented by FE in FIG. 32 utilizing the astigmatism method as mentioned before, and the recording signal Rf can be obtained by the light intensity (Rf in FIG. 32) itself of the transmission light rays passing through the lens 30.
Hereupon, although Lee et al. took the initiative in divising the pick-up (PU) employing the diffraction grating, Sharp Co., Ltd. put it to practical use. The practical structure and its function of the PU are desribed hereinafter as an example referring to the document "Sharp Technical Report No.42/1984, P45."
FIG. 33 shows the structure of the PU. In FIG. 33, the outgoing light rays emitted from LD5 are divided into three light beams; two sub-beams for tracking and a main-beam for reading out the information signal, by use of the diffraction grating for creating the tracking beam which is formed on the rear surface of the diffraction grating 4.
And further, the outgoing light rays emitted from LD5 pass through the diffraciton grating 4 of the upper surface as the 0(zero)-order light rays and are focused on the optical disk 1 by the objective lens 2, after being converted to the parallel light rays by use of the collimation lens 3.
On the other hand, the reflected light rays modulated by the pit on the optical disk 1 are diffracted by the diffraction grating 4, after passing through the objective lens 2 and the collimation lens 3. The diffracted light rays are guided onto the five-divisional PD 6 as the 1-order light rays. The diffraction grating 4 is constructed with two areas having two different grating frequencies respectively. The reflected light rays of the main-beam entering one area thereof as the incident light rays are focused on the divisional line of the light-detecting portions D.sub.2 and D.sub.3, while the reflected light rays of the main-beam entering another area thereof also as the incident light rays are focused on the light-detecting portion D.sub.4. And further, the reflected light rays of the sub-beam are respectively focused on the light-detecting portions D.sub.1 and D.sub.5. Those focused light rays vary as shown in FIG. 34 in accordance with the light-focusing condition of the beam on the optical disk 1.
Consequently, assuming that the outputs of the respective segments of the five-divisional PD 6 are, respectively, S.sub.1, S.sub.2, S.sub.3, S.sub.4, and S.sub.5, the focus error signal Fo is expressed by the following equation (4), utilizing the knife-edge method: EQU Fo=S.sub.2 -S.sub.3 ( 4)
On the other hand, the tracking error signal is detected by the so-called three-beams method. Since the sub-beam for tracking are respectively focused on the light-detecting protions D.sub.1 and D.sub.5, the tracking error signal Tr is expressed by the following equation (5): EQU Tr=S.sub.1 -S.sub.5 ( 5)
And further, the recording signal Rf is expressed by the following equation (6): EQU Rf-S.sub.2 +S.sub.3 +S.sub.4 ( 6)
Next, another example of the pick-up (PU) employing the diffraction grating made by NEC is shown in FIG. 35 (Script of Comprehensive All-Japan Meeting of Electronic Information/Communication Academic Society for celebrating the 70th Anniversary of the Founding of the Academic Society, Showa 62/1987, 1014). The PU of NEC employs the knife-edge method as in the case of the above-mentioned diffraction grating made by Sharp. In addition, the diffraction grating of the NEC PU is constructed by the push-pull method.
Regarding the case of the NEC PU, the double knife-edge method of combining two knife-edge methods employed also by Sharp is employed as the focus detecting method.
In such detecting method, the stability of detecting can be increased by combining the two knife-edge methods. In the latter case of NEC, the track signal can be obtained by forming the slit-state diffraction grating.
In addition to the aforementioned prior art, the optical head of the prior art relevant to the above optical pick-up is described hereinafter.
The construction of the conventional optical head is explained, referring to FIG. 37. The light rays emitted from the laser light source 1 are separated into the light ray fluxes of the 0(zero)-order light rays 4a and the .+-.1-order light rays 4b and 4c, by causing the emitted light rays to enter the diffraction grating surface 3 of the holographic grating 2 as the incident light rays.
Those separated light rays fluxes are focused by the objective lens (not shown) and an optical spot is formed on the surface of the optical disk (not shown) to be employed as the optical information recording medium. The 0(zero)-order light rays 4a read out the information on the surface of the disk, and the .+-.1-order light rays 4b and 4c detect the state of the track. Thereafter, the .+-.1-order light rays become the reflected light rays, and enter again the holographic grating 2 as the incident light rays, and thereby the light rays are separated into the transmission (passing-through) light rays directed to the laser light source 1 and the diffraction light rays directed to the light-receiving element 7. On this occasion, the diffraciton light rays are separated into two groups of the 1-order diffraction light rays; 5a, 5b, and 5c and the other 1-order diffraction light rays; 6a, 6b, and 6c, and all of those six light rays are guided to six light-receiving surfaces a.about.f of the light-receiving elements 7. FIGS. 38a through 38c show, respectively, the shapes of the light spots on the surface of the light-receiving elements 7.
FIG. 38b shows the shapes of the light spots when the disk surface is located at the focus position. FIG. 29a shows the shapes thereof when the disk surface is located at the place nearer than the focus position, while FIG. 29c shows the shapes thereof when the disk surface is located at the place more distant than the focus position.
Hereupon, the focus error signal Fe is detected utilizing the Wedge-prism Method, and the track error signal Te and the reproduction signal Rf are respectively detected utilizing the three-beams method. The calculation equations are as follows: EQU Fe=(a+d)-(b+c) EQU Te=e-f EQU Rf=a+b+c+d
As mentioned above, the optical head is constructed such that the laser light source 1 and the light-receiving element 7 are disposed on the same-level plane and the holographic grating 2 is employed for making the optical head. In such construction, it is possible to perform stable signal detection with the small-sized and low-cost optical head.
Concerning the aforementioned optical pick-up, the output light rays emitted from the semiconductor laser (LD) are radiated onto the optical disk through the diffraction grating, and the reflection light rays reflected on the optical disk enter again the diffraction grating as the incident light rays. Thereby, the focus signal, the track signal, and the recording signal, etc. are detected.
Consequently, as to the conventional optical pick-up, the rate of radiating the light rays onto the optical disk in the diffraction grating is almost 70%. Regarding the light rays detected by the photo-diode (PD) among the reflection light rays reflected on the optical disk, since its diffraction efficiency is almost 10%, the utilization efficiency of the light rays emitted from the light source (LD) is only 0.7.times.0.1=7%. Therefore, the utilization efficiency of the light rays is considerably lowered. This is the defect of the prior-art optical pick-up.
In the conventional optical head as shown in FIG. 37, the holographic grating 2 has only a function of causing the light rays to branch off. Therefore, since some of the diffraction light rays among the light rays emitted from the laser light source 1 and entering the holographic grating 2 as the incident light rays are not radiated onto the optical disk, or some of the transmission light rays reflected on the disk surface and entering again the holographic grating 2 as the incident light rays and passing therethrough are not guided to the light receiving element 7, the efficiency of utilizing the light rays turns out to be worse.
Such phenomenon of lowering the efficiency of utilizing the light rays does not cause any serious problems for the optical head (CD, LD, etc.) specially employed for reproducing only. However, in the case of employing the additionally-writing-in-type of optical head and/or the rewriting-type of optical head, when the efficiency of utilizing the light rays is low, sufficient power of the light rays cannot be obtained on the disk surface at the time of recording on some occasions, or it is necessary to use high-cost and high-power laser light source in order to obtain such sufficient power of the light rays. It follows that the above matters go against the attempt to realize a small-sized and low-cost optical head.